1. Field of the Invention
The present invention relates to a power supply system provided with a plurality of chargeable and dischargeable power storage units, and a vehicle including the power supply system, as well as a control method for the power supply system.
2. Description of the Background Art
The recent interest in environmental issues has drawn people's attention to vehicles having an electric motor as a source of drive force, such as electric vehicle, hybrid vehicle, fuel cell vehicle and the like. Such vehicles are mounted with a power storage unit formed of a secondary battery or electrical double layer capacitor for supplying electric power to the electric motor, or converting kinetic energy generated when regenerative braking is applied into electrical energy for storing the resultant electric power.
For such a vehicle having an electric motor as a source of drive force, it is desirable to increase the charge and discharge capacities of the power storage unit for the purpose of improving the travel performance such as acceleration performance and continuous travel distance, for example. A configuration including a plurality of power storage units has been proposed as a method for increasing the charge and discharge capacities of the power storage units.
Japanese Patent Laying-Open No. 2008-109840 for example discloses a vehicle mounted with a power supply system including: a plurality of power storage units connected in parallel with each other relative to one converter; and a plurality of system main relays provided in association with these power storage units for electrically connecting/disconnecting the associated power storage units and the converter to/from each other, respectively. According to the disclosure, a control device for controlling the converter controls the operation of these system main relays such that the power storage units are used in a specified order and the order of use is changed each time the vehicle system is activated.
Regarding the vehicle having an electric motor as a source of drive force, it has been proposed to electrically connect a power storage unit mounted on the vehicle via a connector to an external power supply such as commercial power supply, such that the power storage unit can be charged by the external power supply. The vehicle in which the power storage unit is thus charged in advance by the external power supply can travel with the internal combustion engine kept stopped, as long as the distance over which the vehicle travels is relatively short, in such cases as commuting or shopping. Accordingly, the overall fuel consumption efficiency can be improved. This charging of the power storage unit by an external power supply is also referred to as “external charging mode,” relative to “travel control mode” for causing the vehicle to travel.
Regarding the vehicle using a plurality of power storage units charged by an external power supply, if the power storage unit to be used first is changed each time the vehicle system is activated as disclosed in the above-referenced Japanese Patent Laying-Open No. 2008-109840, it cannot be ensured that a sub power storage unit used first in the preceding execution of the travel control mode is different from a sub power storage unit to be used first in the current execution of the travel control mode, since the vehicle system is activated each time the travel control mode is performed and the external charging mode is performed. Accordingly, there is a possibility that a specific power storage unit is always used first each time the travel control mode is executed, while the remaining power storage units with a high SOC are left as they are for the period in which the specific power storage unit is used.
The rate of deterioration of the power storage unit considerably varies depending to the environment in which the power storage unit is used. The rate of deterioration of the power storage unit is higher when the power storage unit keeps a high SOC than when the power storage unit is being charged or discharged. In this case, therefore, the rate of deterioration of the remaining power storage units with the SOC kept high is accelerated. A resultant problem is that respective degrees of deterioration of the power storage units are different from each other.